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| United States Patent |
6,180,704
|
|
Takuman
, et al.
|
January 30, 2001
|
Heat-curable silicone rubber composition
Abstract
A Heat-curable silicone rubber composition comprising
(A) 100 weight parts organopolysiloxane with the average unit formula
R.sub.X SiO.sub.(4-x)/2 each R is independently selected from the group
consisting of hydroxyl and substituted and unsubstituted monovalent
hydrocarbon groups and x has a value from 1.9 to 2.1 and containing at
least 2 silicon-bonded alkenyl groups in each molecule,
(B) 10 to 100 weight parts reinforcing filler,
(C) 0.1 to 10 weight parts carboxiloxane dendrimer that contains at least 3
silicon-bonded hydrogen atoms in each molecule, and
(D) 0.1 to 10 weight parts organoperoxide.
| Inventors:
|
Takuman; Osamu (Chiba Prefecture, JP);
Yoshitake; Makoto (Chiba Prefecture, JP)
|
| Assignee:
|
Dow Corning Toray Silicone Co., Ltd. (Tokyo, JP)
|
| Appl. No.:
|
337563 |
| Filed:
|
June 22, 1999 |
Foreign Application Priority Data
| Jun 25, 1998[JP] | 10-194948 |
| Current U.S. Class: |
524/267; 523/466; 524/588; 528/24; 528/32; 556/431; 556/465 |
| Intern'l Class: |
C08L 083/00 |
| Field of Search: |
523/466
524/860,730,731,588
556/459,431,465
528/32,24
|
References Cited
U.S. Patent Documents
| 6077500 | Jun., 2000 | Dvornic et al. | 525/424.
|
| Foreign Patent Documents |
| 976775 | Feb., 2000 | EP.
| |
| 9-95612 | Apr., 1997 | JP.
| |
| 10-166423 | May., 1998 | JP.
| |
| 11-343347 | Dec., 1999 | JP.
| |
Primary Examiner: Dawson; Robert
Assistant Examiner: Zimmer; Marc S.
Attorney, Agent or Firm: Boley; William F.
Claims
We claim:
1. A heat-curable silicone rubber composition comprising
(A) 100 weight parts organopolysiloxane with an average unit formula
R.sub.X SiO.sub.(4-x)/2 containing at least 2 silicon-bonded alkenyl
groups in each molecule, where each R is independently selected from the
group consisting of hydroxyl and substituted and unsubstituted monovalent
hydrocarbon groups and x has a value from 1.9 to 2.1,
(B) 10 to 100 weight parts reinforcing filler,
(C) 0.1 to weight parts carbosiloxane dendrimer that contains at least 3
silicon-bonded hydrogen atoms in each molecule, and
(D) 0.1 to weight parts organoperoxide.
2. The heat-curable silicone rubber composition of claim 1, where the
carbosiloxane dendrimer contains at least one siloxane unit with the
general formula
X.sup.1 R.sup.1.sub.a SiO.sub.(3-a)/2,
where R.sup.1 is a C.sub.1 to C.sub.10 alkyl or an aryl, a is an integer
from 0 to 2, and X.sup.1 is a silylalkyl group with the following formula
at i=1
##STR14##
R.sup.1 is a C.sub.1 to C.sub.10 alkyl or an aryl, R.sup.2 is a C.sub.2 to
C.sub.10 alkylene, R.sup.3 is a C.sub.1 to C.sub.10 alkyl, X.sup.i+1 is
the above-defined silylalkyl group at i=i+1 or the hydrogen atom, i is an
integer with a value from 1 to 10 that specifies the generation of the
silylalkyl group, and b.sup.i is an integer from 0 to 3 with the proviso
that b.sup.1 in at least one X.sup.1 in each molecule is an integer from 0
to 2; wherein when more than 1 of the siloxane units is present they may
be the same or different, and the dendrimer has for its core a
polysiloxane structure of at least 2 silicon atoms that contains the
aforesaid siloxane unit(s) and contains at least 3 silicon-bonded hydrogen
atoms in each molecule.
3. The heat-curable silicone rubber composition of claim 1 where the
carbosiloxane dendrimer is described by average formula
##STR15##
4. The heat-curable silicone rubber composition of claim 1 where the
carbosiloxane dendrimer is described by average formula
##STR16##
5. The heat-curable silicone rubber composition of claim 1 where the
carbosiloxane dendrimer is described by average formula
##STR17##
6. The heat-curable silicone rubber composition of claim 1 where the
carbosiloxane dendrimer is selected from the group consisting essentially
of carbosiloxane dendrimers having the following general formulas
##STR18##
Description
BACKGROUND OF INVENTION
This invention is heat-curable silicone rubber compositions. More
particularly, this invention is heat-curable silicone rubber compositions
that cure rapidly to give very mechanically strong, nondiscoloring
silicone rubber.
The silicone rubber compositions known as millable silicone rubbers as a
general rule cure in the presence of organoperoxide and heat to give
highly heat-resistant, highly weather-resistant silicone rubbers that have
excellent electrical properties, and this good property set has resulted
in the frequent use of these compositions in applications where these
properties are critical. This notwithstanding, a drawback to silicone
rubber compositions of this type has been their slow cure rate. There have
already been attempts at raising the cure rate of these silicone rubber
compositions. For example, Japanese Laid Open (Kokai or Unexamined) Patent
Application Number Hei 9-95612 (95,612/1997) teaches a cure rate-enhancing
method in which dimethylsiloxy-endblocked
dimethylsiloxane-methylhydrogensiloxane copolymer is blended into a
silicone rubber composition that can be cured by
2,5-dimethyl-2,5-di(tert-butylperoxy)hexane.
However, the silicone rubber moldings afforded by this method have a
pronounced tendency to stick to the mold or die and thus suffer from poor
mold-release properties. Thick silicone rubber moldings prepared by this
method also fail to undergo cure in all sections, which prevents the
production of silicone rubber moldings of uniform mechanical strength.
As a result of extensive investigations directed to solving the problems
described above, the inventors discovered that these problems could be
solved by the admixture of a specific type of carbosiloxane dendrimer into
organoperoxide-curing silicone rubber compositions. More specifically, the
object of the present invention is to provide a heat-curable silicone
rubber composition that cures rapidly to give very mechanically strong,
nondiscoloring silicone rubber.
SUMMARY OF INVENTION
A Heat-curable silicone rubber composition comprising
(A) 100 weight parts organopolysiloxane with the average unit formula
R.sub.X SiO.sub.(4-x)/2 containing at least 2 silicon-bonded alkenyl
groups in each molecule, where each R is independently selected from the
group consisting of hydroxyl and substituted and unsubstituted monovalent
hydrocarbon groups and x has a value from 1.9 to 2.1,
(B) 10 to 100 weight parts reinforcing filler,
(C) 0.1 to 10 weight parts carbosiloxane dendrimer that contains at least 3
silicon-bonded hydrogen atoms in each molecule, and
(D) 0.1 to 10 weight parts organoperoxide.
DESCRIPTION OF INVENTION
The present invention is a heat-curable silicone rubber composition
comprising
(A) 100 weight parts organopolysiloxane with an average unit formula
R.sub.X SiO.sub.(4-x)/2 containing at least 2 silicon-bonded alkenyl
groups in each molecule, where each R is independently selected from the
group consisting of hydroxyl and substituted and unsubstituted monovalent
hydrocarbon groups and x has a value of from 1.9 to 2.1,
(B) 10 to 100 weight parts reinforcing filler,
(C) 0.1 to 10 weight parts carbosiloxane dendrimer that contains at least 3
silicon-bonded hydrogen atoms in each molecule, and
(D) 0.1 to 10 weight parts organoperoxide.
To explain the preceding in greater detail, the organopolysiloxane (A),
which is the base component of the present composition, is defined by the
average unit formula R.sub.X SiO.sub.(4-x)/2. The group R in this formula
can be the hydroxyl group or a substituted or unsubstituted monovalent
hydrocarbon group as exemplified by alkyl such as methyl, ethyl, propyl,
butyl, and octyl; alkenyl such as vinyl, allyl, butenyl, and hexenyl; aryl
such as phenyl; 3,3,3-trifluoropropyl; 2-phenylethyl; and 2-cyanoethyl.
The subscript x is a number from 1.9 to 2.1. Each molecule of this
component must contain at least 2 silicon-bonded alkenyl groups. This
alkenyl group can be bonded in pendant or chain terminal position or in
both positions. Component (A) can have a straight-chain or partially
branched straight-chain molecular structure. While the viscosity of
component (A) is not crucial, a preferred viscosity at 25.degree. C. is
from 1,000 to 20,000,000 mPa.s. Component (A) can be a homopolymer,
copolymer, or a mixture of these polymers. The siloxy units making up
component (A) are exemplified by the dimethylsiloxy unit,
methylvinylsiloxy unit, and 3,3,3-trifluoropropylmethylsiloxy unit. The
group present at the molecular terminals of component (A) are exemplified
by trimethylsiloxy, silanol, dimethylvinylsiloxy, and
methylvinylhydroxysiloxy. The organopolysiloxane under consideration can
be exemplified by dimethylvinylsiloxy-endblocked
dimethylsiloxane-methylvinylsiloxane copolymers,
dimethylvinylsiloxy-endblocked dimethylpolysiloxanes, silanol-endblocked
dimethylsiloxane-methylvinylsiloxane copolymers, and
methylvinylhydroxysiloxy-endblocked dimethylsiloxane-methylvinylsiloxane
copolymers.
The reinforcing filler (B) used in the present composition is an essential
component that imparts high levels of mechanical strength to the silicone
rubber moldings afforded by cure of the composition. This reinforcing
filler is exemplified by dry-process silicas such as fumed silicas and by
wet-process silicas such as precipitated silicas and by the
microparticulate silicas afforded by hydrophobicizing the surface of a
silica as listed above with an organosilicon compound such as an
organosilane, organosilazane, organopolysiloxane, or
diorganocyclopolysiloxane. This component should have a particle diameter
no greater than 50 .mu.m and preferably has a specific surface area of at
least 50 m.sup.2 /g and more preferably of at least 100 m.sup.2 /g.
Component (B) is admixed at from 10 to 100 weight parts of component (A)
per 100 weight parts component (A). The use of less than 10 weight parts
will result in an inadequate mechanical strength after cure of the
composition, while at more than 100 weight parts it becomes quite
difficult to blend component (B) into component (A).
The SiH-functional carbosiloxane dendrimer (C) used in the present
composition is the component that characterizes this composition and must
contain at least 3 silicon-bonded hydrogens in each molecule. As used
herein, SiH-functional carbosiloxane dendrimer denotes a
high-molecular-weight compound with a predictable and highly branched
structure that elaborates radially from a single core. Carbosiloxane
dendrimers with such a structure are described in detail, for example, in
Japanese Patent Application Number H10-166423 (166,423/1998), filed May
29, 1998, to Dow Corning Toray Silicone Co., Ltd. Among the therein
described carbosiloxane dendrimers, the carbosiloxane dendrimers preferred
for the present invention contains in each molecule at least 3 SiH and at
least one siloxane unit with the general formula
X.sup.1 R.sup.1.sub.a SiO.sub.(3-a)/2,
where R.sup.1 is a C.sub.1 to C.sub.10 alkyl or an aryl, a is an integer
from 0 to 2, and X.sup.1 is a silylalkyl group with the following formula
at i=1
##STR1##
R.sup.1 is a C.sub.1 to C.sub.10 alkyl or an aryl, R.sup.2 is a C.sub.2 to
C.sub.10 alkylene, R.sup.3 is a C.sub.1 to C.sub.10 alkyl, X.sup.i+1 is
the above-defined silylalkyl group at i=i+1 or the hydrogen atom, i is an
integer with a value from 1 to 10 that specifies the generation of the
silylalkyl group, and b.sup.i is an integer from 0 to 3 with the proviso
that b.sup.1 in at least one X.sup.1 in each molecule is an integer from 0
to 2; wherein when more than 1 is present the subject siloxane units may
be the same or different, and has for its core a polysiloxane structure of
at least 2 silicon atoms that contains the aforesaid siloxane unit(s).
R.sup.1 in the preceding formulas used to specify the preferred
carbosiloxane dendrimers is a C.sub.1 to C.sub.10 alkyl or an aryl. The
alkyl encompassed by R.sup.1 can be exemplified by methyl, ethyl, propyl,
butyl, pentyl, isopropyl, isobutyl, cyclopentyl, and cyclohexyl, while the
aryl encompassed by R.sup.1 can be exemplified by phenyl and naphthyl.
Methyl is preferred among the preceding. The subscript a is an integer
with a value from 0 to 2. X.sup.1 is the silylalkyl group with the
following formula at i=1.
##STR2##
R.sup.1 in this formula is defined as above. R.sup.2 in the preceding
formula is a C.sub.2 to C.sub.10 alkylene, for example, straight-chain
alkylene such as ethylene, propylene, butylene, and hexylene and branched
alkylene such as methylmethylene, methylethylene, 1-methylpentylene, and
1,4dimethylbutylene. Ethylene, methylmethylene, hexylene,
1-methylpentylene, and 1,4-dimethylbutylene are preferred for R.sup.2.
R.sup.3 is a C.sub.1 to C.sub.10 alkyl, for example, methyl, ethyl,
propyl, butyl, pentyl, and isopropyl, among which methyl and ethyl are
preferred. X.sup.i+1 is the hydrogen atom or the above-defined silylalkyl
group. i is an integer from 1 to 10 that indicates the number of
generations of the silylalkyl group under consideration, i.e., it
indicates the number of repetitions of this silylallyl group. The
subscript b.sup.i is an integer from 0 to 3 wherein b.sup.1 in at least
one X.sup.1 in each molecule is an integer from 0 to 2. Thus, this
silylalkyl group has the following general formula when the number of
generations is 1:
##STR3##
the following general formula when the number of generations is 2:
##STR4##
and has the following general formula when the number of generations is 3.
##STR5##
The polysiloxane structure having at least 2 silicon atoms that is present
in this carbosiloxane dendrimer must contain at least 1 siloxane unit as
represented by the general formula X.sup.1 R.sup.1.sub.a SiO.sub.(3-a)/2
wherein when more than 1 is present the subject siloxane units may be the
same or different. The structural units in this organosiloxane comprise
monofunctional siloxane units (M units) with the general formulas X.sup.1
R.sub.1.sub.2 SiO.sub.1/2 and R.sup.1.sub.3 SiO.sub.1/2, difunctional
siloxane units (D units) with the general formulas X.sup.1 R.sup.1
SiO.sub.2/2 and R.sup.1.sub.2 SiO.sub.2/2, trifunctional siloxane units (T
units) with the general formulas X.sup.1 SiO.sub.3/2 and R.sup.1
SiO.sub.3/2, and the tetrafunctional siloxane unit (Q unit) SiO.sub.4/2.
The polysiloxane under consideration can be more specifically exemplified
by the following general formulas, in which X.sup.1 and R.sup.1 are
defined as above and m, n, x, y, z, p, q, r, s, and t, which denote the
number of siloxane units present in each molecule, have values .gtoreq.1
wherein p+q.gtoreq.5 and s+t.gtoreq.6.
##STR6##
The carbosiloxane dendrimer (C) can be a single compound or a mixture of
compounds. The dispersity index of the molecular weight (polystyrene
basis), that is, weight-average molecular weight/number-average molecular
weight (M.sub.w /M.sub.n), is preferably .ltoreq.2. The subject
carbosiloxane dendrimer can be specifically exemplified by polymers with
the following average molecular formulas.
##STR7##
The organoperoxide (D) used in the present composition functions to cure
the composition. This organoperoxide can be exemplified by benzoyl
peroxide; methyl-substituted benzoyl peroxides such as
bis(ortho-methylbenzoyl peroxide), bis(meta-methylbenzoyl peroxide),
bis(para-methylbenzoyl peroxide), 2,3-dimethylbenzoyl peroxide,
2,4-dimethylbenzoyl peroxide, 2,6-dimethylbenzoyl peroxide,
2,3,4-trimethylbenzoyl peroxide, and 2,4,6-trimethylbenzoyl peroxide;
tert-butyl perbenzoate; dicumyl peroxide; and
2,5-dimethyl-2,5-di(tert-butylperoxy)hexane. The organoperoxide is added
generally at from 0.1 to 10 weight parts and preferably at from 1 to 5
weight parts, in each case per 100 weight parts component (A).
The present composition comprises the components (A) to (D) as described
above, but may also contain, insofar as the object of the present
invention is not impaired, the various additives known in the art for
addition to silicone rubber compositions. These additives can be generally
exemplified by nonreinforcing fillers, inorganic pigments, heat
stabilizers, flame retardants, internal release agents, and plasticizers.
The nonreinforcing fillers can be specifically exemplified by diatomaceous
earth, quartz powder, calcium carbonate, mica, talc, magnesium oxide,
aluminum oxide, aluminum hydroxide, and magnesium hydroxide. The inorganic
pigments can be specifically exemplified by iron oxide red and titanium
dioxide. The heat stabilizer can be exemplified by rare earth oxides, rare
earth hydroxides, cerium silanolate, and the fatty acid salts of cerium.
The flame retardants can be exemplified by platinum, platinum compounds,
benzotriazole, fumed titanium dioxide, manganese carbonate, and zinc
carbonate. The internal release agents can be exemplified by fatty acid
salts such as calcium stearate.
The present composition can be prepared simply by mixing components (A) to
(D) to homogeneity in their specified proportions or by mixing the
optional additive components to homogeneity with components (A) to (D) in
their specified proportions. The means for intermixing components (A) to
(D) and the optional additive components can be those mixing devices and
mixing/kneading devices heretofore used for the preparation of silicone
rubber compositions, for example, kneader mixers, twin-screw continuous
extrusion mixers, and two-roll mills.
The present composition can be cured by heating the composition to at least
the decomposition temperature of the organoperoxide (D) while staying
below temperatures at which scorching would occur. In specific terms, this
will be in the range from 130.degree. C. to 200.degree. C. and is
preferably in the range from 140.degree. C. to 170.degree. C. The present
composition can be molded by those methods heretofore known for molding
heat-curing silicone rubber compositions, such as compression molding,
injection molding, and extrusion molding.
The present heat-curable silicone rubber composition has a high cure rate
and therefore can support reductions in the molding cycle time during
molding resulting in major reductions in molding costs. Moreover, the
cured silicone rubber afforded by this composition exhibits excellent
mechanical strength and resists discoloration and is therefore well-suited
for use in those applications were such properties are critical.
The invention will be explained in greater detail below using working
examples, in which parts denotes weight parts. The properties of the
silicone rubbers were measured as follows.
Tensile Strength and Tensile Elongation
Test specimens with a thickness of 2 mm were fabricated by first
press-curing the silicone rubber composition at 170.degree. C. for 10
minutes and then oven-curing at 200.degree. C. for 4 hours. The test
specimens were test for tensile strength and elongation according to JIS
K-6251.
Compression Set
Cylindrical test specimens (diameter=29 mm, thickness=12.5 mm) were
fabricated by first press-curing the silicone rubber composition at
170.degree. C. for 10 minutes and then oven-curing at 200.degree. C. for 4
hours. The test specimens were tested for compression set according to JIS
K-6263.
Cure Rate
The time (T.sub.90) for the torque value to reach 90% of the final torque
value was measured using a Curastometer (model JSR Curastometer-III from
Orientec) at a temperature of 170.degree. C.
Color Change
The color change was evaluated visually.
Reference Example 1. 103.6 g Of vinyltrimethoxysilane and 0.04 g of a 3
weight percent solution of chloroplatinic acid in isopropanol were
introduced into a 200-mL four-neck flask equipped with a stirrer,
thermometer, reflux condenser, and an addition funnel and then heated to
100.degree. C. while stirring. To this was then gradually added 49.4 g of
tetrakis(dimethylsiloxy)silane dropwise from the addition funnel so as to
maintain the reaction temperature at 100 to 110.degree. C. After
completion of the addition, the reaction solution was heated for an
additional 1 hour at 120.degree. C. After cooling, the reaction solution
was transferred to an evaporation flask and concentrated under reduced
pressure on a rotary evaporator to give 138.4 g of a very light brown
liquid. Then, 141.0 g of 1,1,3,3-tetramethyldisiloxane, 100 mL of
concentrated hydrochloric acid, 200 mL of water, and 200 mL of isopropanol
were placed in a 1-liter four-neck flask equipped with a stirrer,
thermometer, reflux condenser, and an addition funnel and were stirred.
80.6 g Of the very light brown liquid prepared as described above was then
gradually added dropwise from the addition funnel over 1 hour. After the
completion of addition, the reaction solution was stirred at room
temperature for an additional 1 hour. The reaction solution was then
transferred to a separatory funnel, the lower layer was separated off, and
the remaining upper layer solution was washed 2 times with 200 mL of
water, then washed once with 50 mL of saturated aqueous sodium bicarbonate
solution, and finally dried over anhydrous magnesium sulfate. The produced
solids were filtered off and the resulting solution was transferred to an
evaporation flask and concentrated under reduced pressure on a rotary
evaporator to yield carbosiloxane dendrimer with the following average
compositional formula.
##STR8##
Reference Example 2. 103.6 g Of vinyltrimethoxysilane and 0.04 g of a 3
weight percent solution of chloroplatinic acid in isopropanol were
introduced into a 200-mL four-neck flask equipped with a stirrer,
thermometer, reflux condenser, and an addition funnel and then heated to
100.degree. C. while stirring. To this was gradually added 49.4 g of
methyltris(dimethylsiloxy)silane dropwise from the addition funnel so as
to maintain the reaction temperature at 100 to 110.degree. C. After
completion of the addition, the reaction solution was heated for an
additional 1 hour at 120.degree. C. After cooling, the reaction solution
was transferred to an evaporation flask and concentrated under reduced
pressure on a rotary evaporator to give 138.4 g of a very light brown
liquid. Then, 141.0 g of 1,1,3,3-tetramethyldisiloxane, 100 mL of
concentrated hydrochloric acid, 200 mL of water, and 200 mL of isopropanol
were placed in a 1-liter four-neck flask equipped with a stirrer,
thermometer, reflux condenser, and an addition funnel and were stirred.
80.6 g Of the very light brown liquid prepared as described above was then
gradually added dropwise from the addition funnel over 1 hour. After the
completion of addition, the reaction solution was stirred at room
temperature for an additional 1 hour. The reaction solution was then
transferred to a separatory funnel, the lower layer was separated off, and
the remaining upper layer solution was washed twice with 200 mL of water,
then washed once with 50 mL of saturated aqueous sodium bicarbonate
solution, and finally dried over anhydrous magnesium sulfate. The produced
solids were filtered off and the resulting solution was transferred to an
evaporation flask and concentrated under reduced pressure on a rotary
evaporator to yield carbosiloxane dendrimer with the following average
compositional formula.
##STR9##
Reference Example 3. 88.9 g Of vinyltrimethoxysilane and 0.04 g of a 3
weight percent solution of chloroplatinic acid in isopropanol were
introduced into a 200-mL four-neck flask equipped with a stirrer,
thermometer, reflux condenser, and an addition funnel and then heated to
100.degree. C. while stirring. To this was gradually added 30.0 g of
1,3,5,7-tetramethylcyclotetrasiloxane dropwise from the addition funnel so
as to maintain the reaction temperature at 100 to 110.degree. C. After
completion of the addition, the reaction solution was heated for an
additional 1 hour at 120.degree. C. After cooling, the reaction solution
was transferred to an evaporation flask and concentrated under reduced
pressure on a rotary evaporator to give 100.4 g of a very light brown
liquid. Then, 93.0 g of 1,1,3,3-tetramethyldisiloxane, 30 mL of
concentrated hydrochloric acid, 60 mL of water, and 60 mL of isopropanol
were placed in a 1-liter four-neck flask equipped with a stirrer,
thermometer, reflux condenser, and an addition funnel and were stirred.
80.0 g Of the very light brown liquid prepared as described above was then
gradually added dropwise from the addition funnel over 1 hour. After the
completion of addition, the reaction solution was stirred at room
temperature for an additional 1 hour. The reaction solution was then
transferred to a separatory funnel, the lower layer was separated off, and
the remaining upper layer solution was washed 2 times with 100 mL of
water, then washed once with 100 mL of saturated aqueous sodium
bicarbonate solution, and finally dried over anhydrous sodium sulfate. The
solids were filtered off and the resulting solution was transferred to an
evaporation flask and concentrated under reduced pressure on a rotary
evaporator to yield carbosiloxane dendrimer with the following average
compositional formula.
##STR10##
Example 1
The following were introduced into a kneader mixer and mixed to
homogeneity: 100 parts dimethylvinylsiloxy-endblocked
dimethylsiloxane-methylvinylsiloxane copolymer gum (99.87 mole %
dimethylsiloxane units and 0.13 mole % methylvinylsiloxane units), 45
parts wet-process silica with a specific surface area of 200 m.sup.2 /g,
and 4.5 parts silanol-endblocked dimethylpolysiloxane with a viscosity of
30 mPa.s. This mixture was then heated and mixed at 175.degree. C. for 60
minutes to give a silicone rubber base compound. A silicone rubber
composition was prepared on a two-roll mill by adding the following with
mixing to homogeneity to 100 parts of this base: 0.5 part of the
carbosiloxane dendrimer (silicon-bonded hydrogen content=0.65 weight %)
with the following average compositional formula as synthesized in
Reference
Example 1
##STR11##
and 0.6 part of a 50 weight percent silicone oil paste masterbatch of
2,5-dimethyl-2,5-di(tert-butylperoxy)hexane.
The silicone rubber composition thus prepared was press-cured to mold a 2
mm-thick silicone rubber sheet for measurement of the physical properties
as described above. These results are reported in Table 1. The cure rate
and color change were also evaluated and these results are likewise
reported in Table 1.
Example 2
The following were introduced into a kneader mixer and mixed to
homogeneity: 100parts dimethylvinylsiloxy-endblocked
dimethylsiloxane-methylvinylsiloxane copolymer gum (99.87 mole %
dimethylsiloxane units and 0.13 mole % methylvinylsiloxane units), 45
parts wet-process silica with a specific surface area of 200 m.sup.2 /g,
and 4.5 parts silanol-endblocked dimethylpolysiloxane with a viscosity of
30 mPa.s. This mixture was then heated and mixed at 175.degree. C. for 60
minutes to give a silicone rubber base compound. A silicone rubber
composition was prepared on a two-roll mill by adding the following with
mixing to homogeneity to 100 parts of this base: 0.5 part of the
carbosiloxane dendrimer (silicon-bonded hydrogen content=0.69 weight
percent ) with the following average compositional formula as synthesized
in Reference
Example 2
##STR12##
and 0.6 part of a 50 weight percent silicone oil paste masterbatch of
2,5-dimethyl-2,5-di(tert-butylperoxy)hexane.
The silicone rubber composition thus prepared was press-cured to mold a 2
mm-thick silicone rubber sheet for measurement of the physical properties
as described above. These results are reported in Table 1. The cure rate
and color change were also evaluated and these results are likewise
reported in Table 1.
Example 3
The following were introduced into a kneader mixer and mixed to
homogeneity: 100 parts dimethylvinylsiloxy-endblocked
dimethylsiloxane-methylvinylsiloxane copolymer gum (99.87 mole %
dimethylsiloxane units and 0.13 mole % methylvinylsiloxane units), 45
parts wet-process silica with a specific surface area of 200 m.sup.2 /g,
and 4.5 parts silanol-endblocked dimethylpolysiloxane with a viscosity of
30 mPa.s. This mixture was then heated and mixed at 175.degree. C. for 60
minutes to give a silicone rubber base compound. A silicone rubber
composition was prepared on a two-roll mill by adding the following with
mixing to homogeneity to 100 parts of this base: 0.5 part of the
carbosiloxane dendrimer (silicon-bonded hydrogen content=0.72 weight
percent) with the following average compositional formula as synthesized
in Reference Example 3
##STR13##
and 0.6 part of a 50 weight percent silicone oil paste masterbatch of
2,5-dimethyl-2,5-di(tert-butylperoxy)hexane.
The silicone rubber composition thus prepared was press-cured to mold a 2
mm-thick silicone rubber sheet for physical properties testing as
described above. These results are reported in Table 1. The cure rate and
color change were also evaluated and these results are likewise reported
in Table 1.
Comparative Example 1
The following were introduced into a kneader mixer and mixed to
homogeneity: 100 parts dimethylvinylsiloxy-endblocked
dimethylsiloxane-methylvinylsiloxane copolymer gum (99.87 mole %
dimethylsiloxane units and 0.13 mole % methylvinylsiloxane units), 45
parts wet-process silica with a specific surface area of 200 m.sup.2 /g,
and 4.5 parts silanol-endblocked dimethylpolysiloxane with a viscosity of
30 mPa.s. This mixture was then heated and mixed at 175.degree. C. for 60
minutes to give a silicone rubber base compound. A silicone rubber
composition was prepared on a two-roll mill by adding 0.6 part of a 50
weight percent silicone oil paste masterbatch of
2,5-dimethyl-2,5-di(tert-butylperoxy)hexane with mixing to homogeneity to
100 parts of this base.
The silicone rubber composition thus prepared was press-cured to mold a 2
mm-thick silicone rubber sheet for measurement of the physical properties
as described above. These results are reported in Table 1. The cure rate
and color change were also evaluated and these results are likewise
reported in Table 1.
Comparative Example 2
The following were introduced into a kneader mixer and mixed to
homogeneity: 100 parts dimethylvinylsiloxy-endblocked
dimethylsiloxane-methylvinylsiloxane copolymer gum (99.87 mole %
dimethylsiloxane units and 0.13 mole % methylvinylsiloxane units), 45
parts wet-process silica with a specific surface area of 200 m.sup.2 /g,
and 4.5 parts silanol-endblocked dimethylpolysiloxane with a viscosity of
30 mPa.s. This mixture was then heated and mixed at 175.degree. C. for 60
minutes to give a silicone rubber base compound. A silicone rubber
composition was prepared on a two-roll mill by adding the following with
mixing to homogeneity to 100 parts of this base: 0.5 part
organohydrogenpolysiloxane (silicon-bonded hydrogen content=0.70 weight
percent) and 0.6 part of a 50 weight percent silicone oil paste
masterbatch of 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane. The properties
of this composition were measured and evaluated as in Example 1 and the
results are reported in Table 1.
TABLE 1
Comp. Comp.
Example 1 Example 2 Example 3 Example 1 Example 2
curability 80 77 75 186 98
T.sub.90
seconds
durometer 51 51 51 52 51
tensile 7.9 8.0 8.2 8.5 8.0
strength MPa
elongation % 310 320 320 330 310
color change no no no present no
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